The speedo on the CB250RS I borrowed back in the 90s and on the one I own now are not especially accurate, so I'm wondering how accurate my tacho is. My sprockets are 14 front and 44 rear, which I believe is standard. At an indicated 100 km/h, which is actually a bit over 105, my tacho indicates about 5,000 rpm. How does that compare with other people's CB250RSs? If my tacho is as inaccurate as my brother's speedo was, then by the time it indicated 8,500 rpm, the motor would actually be doing 10,000 rpm.
If they are anything like the mc22 I've read its about 5% out with stock sprockets. I've also read its an expensive wild goose chase to correct and most people opt out and go with gps.
Tacho gets its imput from the cdi and I believe that the tacho's themselves are a little inaccurate. You would be able to test this but how much it is out is anyones guess. This guy tested his tacho with a frequency generator "http://cbr250.com/forums/maintenance/277-tacho-accurate.html"
Follow a car going 60km/hr (use a mate/family whatever) and see what yours reads. Easiest way to check. Not sure where your cable is hooked to but I'd guess it's the front wheel so it's just converting your average tyre rotation at a certain velocity to a speed on the dial. For the tacho, well... don't know. I think that would run off the current supplied to the engine...? Not sure how a cable one would work.... connected to drive shaft?
I have checked the speedo by comparing it with my GPS. I know the exact extent of the speedo error. It is the tacho that I am wondering about. As an old fuddy duddy/grumpy old man/old fart, I'm amused by all you young whipper snippers assuming that the speedo is electronic. For those too young to have ever seen old technology, there was a time when tachos had a cable and worked in exactly the same way as (old fashioned) speedos. They were often driven off the camshaft, possible because that was up near the top of the motor and allowed for a reasonable short tacho cable. I was hoping that someone had either done the necessary maths to work out how many kilometres per hour a 1980 CB250RS should do per thousand revs. Or that at least someone else with the same model could tell me what revs their bike does at 100km/h. However, I do appreciate that people are taking the time to try to help me.
As another old fuddy duddy/grumpy old man/old fart .... I was mildly amused also Maybe all the gen Ys should also google 'kick starter' @TechHeadFred is your man ..... I think he sleeps with one of his CB250s
I don't exactly sleep with one of them but I do have three and literally grew up on the back of one! Not sure on testing tacho accuracy off the top of my head but I'll have a look-see in the Haynes and get back to you if it has any tips.
You could do an engineering calc which converts the RPM of your shaft through the gear ratio (front sprocket to rear sprocket) and get some sort of indicative rotational speed at the wheel. Unfortunately you'd have to find a non-lazy engineer for that...
Checking details on Gearing Commander based on specs from the manual and tyre size conversion charts. The XL250 is in their database, so I used it as a starting point and adjusted the values accordingly. Here's what it gave me: So your tacho should probably be sitting on about 6500 in 5th gear when you are doing 105 km/h.
You can buy a handheld tachometer for peanuts off ebay. Stick a piece of reflective tape on your flywheel and check the rpm. Then check the manual to determine your primary drive ratio, the five gearbox ratios, and the final ratio by counting teeth on your countershaft and rear wheel sprocket. Lastly make a pen mark on the side of your tyre and adjacent to that on your driveway. Walk your bike forward until the rear wheel makes one rotation and mark the spot again. Then measure with tape measure. Now you have tyre circumference. Crank rpm * Primary Ratio = Primary Shaft rpm Primary Shaft rpm * Gear Ratio = Countershaft rpm Countershaft rpm * Final Ratio = Wheel rpm Wheel rpm * Tyre Circumference(in km) / 60 = km per hour cheers Blair
Thanks for that. It seems my concerns about the accuracy of the tacho were justified. I had searched in the manual available on this forum for the information needed to do the calculations you have done, but I had made the mistake of assuming that the right hand side of page six, like so many others was a foreign language version of what was on the left. As I was zoomed in to show just the left hand side of the page, I missed the gearing specifications on the right. I am embarrassed that if I had looked a bit more carefully, I could have done the calculations myself. I just noticed the tyre size you entered. Mine has what I believe is the standard 110/90, which gives a circumference fairly close to what my tape measure gives. That suggests that an indicated 100 km/h is a bit over 6000 rpm.
Yeah the tyre size was hard to convert from the imperial stock sizes in the manual. Best guesses were 90/90-18 and 100/80-18.
Sit on the bike when you do the circumference measuring as the dia will be slightly different from unloaded.
